The present invention relates to a control device which is used to control the position of the leading end of an actuator.
Referring to FIG. 4, there is shown a block diagram of a two-stage actuator tracking control system of an optical disc device which is published under the title of "A TWO-STAGE COUPLING SERVO SYSTEM FOR AN OPTICAL DISC MEMORY" in the theses of the optical memory symposium '85, pp. 203 to 208. In FIG. 4, the sum of a positioning signal 13 of a tracking actuator 3 operable at high frequencies in a small range, and a positioning signal 16 of a linear motor 8 operable at low frequencies in a large range provides a positioning signal 10 of an optical spot. A difference between a target positioning signal 9 and the optical spot positioning signal 10 provides a track shift quantity (control deviation) 11 and is then input to a stabilizing compensator 1 included in a two-stage coupling servo system. An output signal 12 of the stabilizing compensator 1 is input through a tracking actuator drive circuit 2 to the tracking actuator 3. Also, the output signal 12 of the stabilizing compensator 1 is input to a coupling compensator 41, which is adapted to allow mainly low frequencies to pass therethrough, through a linear motor drive circuit 7 to the linear motor 8. That is, a low frequency component of a control deviation 11 is transmitted to the linear motor 8, while a high frequency component of the control deviation 11 is transmitted to the tracking actuator 3.
In FIG. 5, there are shown a transfer characteristic 51 which is obtained by a combination of the tracking actuator drive circuit 2 and tracking actuator 3 and a transfer characteristic 52 which is obtained by a combination of the coupling compensator 41, linear motor drive circuit 7 and linear motor 8. As can be understood from FIG. 5, with .omega..sub.c as the boundary, the operating region of the tracking actuator 3 is separated from the operating region of the linear motor 8. However, at .omega..sub.1 as well, the gain of the transfer characteristic 51 is equal to that of 52 and a phase difference between the two transfer characteristics 51 and 52 is near to 180.degree..
For this reason, in the above-mentioned conventional two-stage actuator control device, if the device is vibrated at about frequencies of .omega..sub.1, then the tracking actuator 3 and linear motor 8 are vibrated at a mutually equal amplitude and in substantially opposite directions to thereby increase the amplitude of the device too greatly, so that the two-stage coupling servo system becomes destabilized.